Incorporation of hexafluoroisopropylidene(6F) units into a polymer backbone structure assists in the processing of high temperature resistant aromatic heterocyclic polymer systems. These 6F units tend to lower the glass transition temperature (Tg) of polymers prepared therewith and thus improve their melt characteristics. In addition, 6F units within a polymer chain prohibit extensive conjugation of aromatic moieties, thereby providing good electric insulation characteristics for the end product resins. Such lack of conjugation also obliterates UV-visible chromophores, thus allowing end product polyimides to appear colorless and to resist photochemical degradation.
Key monomers for the synthesis of thermally stable high temperature resins containing 6F groups have been prepared from commercially available 2,2-bis(4hydroxyphenyl)hexafluoropropane, commonly known as Bisphenol AF. However, because of the resistance of the hydroxyl functions of Bisphenol AF to replacement by direct chemical modification, the availability of such monomers is severely limited to monomers having substituents which are less reactive than halogen substituents.
Poly(arylethers) and poly(arylthioethers) having as their base monomer units diphenylhexafluoropropanes connected by either oxygen or sulfur linkages are particularly useful polymers, with outstanding mechanical, optical and electrical properties. One method of synthesizing poly(arylethers) and poly(arylthioethers) involves a nucleophilic attack on the para-carbon on the phenyl rings of diphenyl hexafluoropropane. A leaving group, such as one of various halogens, is present on the para-carbon and is displaced during the nucleophilic attack. Of the various halogenated diphenylhexafluoropropanes, difluorinated diphenylhexafluoropropane [2,2-bis(4-fluorophenyl) hexafluoropropane] is the only one that is truly effective because other halogen substituted diphenyl hexafluoropropane compounds do not possess the same reactivity for the desired nucleophilic aromatic substitution reaction in the polymer synthesis.
One apparent method of synthesizing 2,2-bis (4-fluorophenyl)hexafluoropropane is described in U.S. Pat. No. 4,503,254 to Kelleghan, assigned to the same assignee as the present application. The process described in the Kelleghan patent, bowever, is primarily used to synthesize bromo-substituted diphenyl hexafluoropropanes. The Kelleghan method is not well suited for synthesizing fluoro-substituted diphenyl hexafluoropropane because in such a synthesis, an absolutely anhydrous condition is essential to the reaction. Any variation in the required anhydrous condition will dramatically lower the yield of fluorinated product. It is thus very difficult to obtain an acceptable yield of 2,2-bis(4-fluorophenyl)hexafluoropropane using Bisphenol AF as a starting material.
The synthesis of poly(arylethers) and poly(arylthioethers) can theoretically be performed using halogen-substituted diphenylhexafluoropropanes other than 2,2-bis(4-fluorophenyl)hexafluoropropane, but the yield is extremely poor because such compounds do not possess the desired reactivity for the required nucleophilic aromatic substitution. What is needed, therefore, is a process for synthesizing poly(arylethers) and poly(arylthioethers) using a monomer having a high reactivity for the required nucleophilic aromatic substitution reaction during such synthesis. What is also needed is a method of synthesizing such a monomer in sufficient yield to make its use in the polymerization reaction feasible. The present invention satisfies these needs and provides other related advantages.